(1) Field of the Invention
The present invention relates to a process for producing a molded glass article and more particularly to a process for producing a molded glass article from a glass material containing lead oxide.
(2) Description of the Prior Art
In recent years, many attempts have been made to press mold an optical lens or the like at a high precision without conducting post-treatments such as polishing and the like. In one of such molding processes, a glass material placed between an upper mold and a lower mold both made of a ceramic or the like is heated to a temperature close to the softening point of said glass and then subjected to press molding. This press molding is effected generally in a non-oxidizing or reducing gas atmosphere such as nitrogen gas or the like. The reason is that the use of a non-oxidizing gas atmosphere can prevent not only the oxidation of the material (e.g. stainless steel) constituting the pressing machine used but also the oxidation of the mold surface (having a mirror surface) which is in contact with the glass material (the mold surface, when the mold is repeatedly used in an oxidizing atmosphere, gets rough and loses the mirror surface smoothness).
However, when a glass material containing, for example, about 20% by weight or more of lead oxide is subjected to press molding in a non-oxidizing atmosphere containing oxygen in a very small amount of 100 ppm or below, the lead oxide (PbO) at the glass surface is chemically reduced to become other lead compound or metallic lead (Pb), and the surface of the molded glass article gets cloudy. When the mold is used repeatedly under such a condition, the mold surface reacts with the lead component of the glass surface to form a reaction product and thereby to lose the mirror surface smoothness. Presently, there has been found no effective method for dissolving the reaction product in order to restore the original mirror surface. Moreover, the molded glass article obtained with the mold whose surface has lost the mirror surface smoothness, has an uneven surface.
In order to solve the above problem encountered when a lead oxide-containing glass is subjected to press molding, Japanese Patent Application Kokai (Laid-Open) No. 162629/1987 discloses a process wherein press molding is effected by changing the oxygen concentration in non-oxidizing atmosphere after every given times of molding.
However, when a lead oxide-containing glass material is subjected to press molding by the process described in the above patent document, neither the cloudiness at the molded glass article surface nor the adhesion of lead to the mold surface can be prevented completely.